Phase separation of an asymmetric binary fluid mixture confined in a nanoscopic slit pore: Molecular-dynamics simulations

摘要

As a generic model system of an asymmetric binary fluid mixture, hexadecanedissolved in carbon dioxide is considered, using a coarse-grained bead-springmodel for the short polymer, and a simple spherical particle with Lennard-Jonesinteractions for the carbon dioxide molecules. In previous work, it has beenshown that this model reproduces the real phase diagram reasonable well, andalso the initial stages of spinodal decomposition in the bulk following asudden expansion of the system could be studied. Using the parallelizedsimulation package ESPResSo on a multiprocessor supercomputer, phase separationof thin fluid films confined between parallel walls that are repulsive for bothtypes of molecules are simulated in a rather large system (1356 x 1356 x 67.8A^3, corresponding to about 3.2 million atoms). Following the sudden systemexpansion, a complicated interplay between phase separation in the directionsperpendicular and parallel to the walls is found: in the early stages thehexadecane molecules accumulate mostly in the center of the slit pore, but asthe coarsening of the structure in the parallel direction proceeds, theinhomogeneity in the perpendicular direction gets much reduced. Studying thenthe structure factors and correlation functions at fixed distances from thewall, the densities are essentially not conserved at these distances, and hencethe behavior differs strongly from spinodal decomposition in the bulk. Some ofthe characteristic lengths show a nonmonotonic variation with time, and simplecoarsening described by power-law growth is only observed if the domain sizesare much larger than the film thickness.